What is the difference between an overload and a short circuit?

B. An overload is normal-path current somewhat above rating for a while; a short circuit is current taking an unintended low-impedance path, many times the rating

A. There is none — both are just "too much current"

C. An overload only happens on motors; a short circuit only on lighting

D. A short circuit is always to ground; an overload is always line to line

On a time-current curve, which way does "more current" run, and which way "more time"?

B. Current across (left to right), time up (bottom to top) — both usually on logarithmic scales

D. Time across, current up, both linear

Why is the curve of an inverse-time breaker or fuse sloped rather than a straight vertical line?

B. So it waits longer on small overloads (like a motor starting) and trips faster the bigger the current gets

C. Because the wire resistance changes

What does "selective coordination" aim for?

B. That only the device nearest the fault opens, so the rest of the system stays on

A. That every breaker in the building trips together, to be safe

C. That the main breaker always trips first

D. That fuses and breakers are never mixed

You lay the branch breaker's curve and the feeder breaker's curve on one sheet. What indicates they are likely to coordinate at a given fault current?

B. At that current the branch device's curve (including its tolerance band) sits entirely below and left of the feeder device's band — the branch clears before the feeder starts to trip

A. The two curves cross at that current

C. The feeder curve is below the branch curve

D. The curves are the same shape

A breaker trips instantly the moment a circuit is switched on, every time, with nothing plugged in. The most likely story is…

B. A short circuit or ground fault on the circuit

Why does a protective device also carry an interrupting rating?

B. To say the most fault current it can safely break; a fault bigger than that can destroy the device instead of clearing

A. To say what load it can carry continuously

C. It is the same as its trip setting

Tercer año Lesson 9 of 13 · about 14 min

Overcurrent protection and coordination

What you’ll learn

The difference between an overload and a short circuit, and why a device treats them differently.
How to read a time-current curve: current across, time up, both on log scales.
Trip rating versus interrupting rating — two different numbers.
What selective coordination means and what it looks like on a curve sheet.

Before this: Second year · The overload relay

Overcurrent protection has one job: open the circuit before too much current does damage — to the conductors, the equipment, or the building. It has to do that fast for a fault and patiently for a surge that is normal, and ideally it should take out only the part of the system that has the problem. Those three demands shape every fuse and breaker. What size of device goes where is your code book’s business; this lesson is about how they behave.

Two kinds of too much

An overload is a healthy circuit asked to carry more than it was built for — too many heaters on one circuit, a motor dragging a jammed conveyor. The current is a little high, the danger is heat building in insulation over minutes, and a short surge (a motor starting) is normal and must not trip anything.

A short circuit or ground fault is current taking a path it was never meant to — line to line, or line to a grounded enclosure. The only thing limiting it is the impedance of the supply and the wiring, so it can be hundreds or thousands of amps. The danger is immediate: melting conductors, arcing, and mechanical forces on busbars. It must be cleared in a fraction of a second.

Reading a time-current curve

A sketch, not a real device. Each curve slopes (the more current, the less time) then drops straight down to an instantaneous region for faults. The branch device sits left of and below the feeder: for most currents it clears first. Where the two flatten out together at the bottom right, a big enough fault could open both.

Current runs across, time up, and both axes are logarithmic so a single sheet can go from a few amps to tens of thousands and from milliseconds to hours. To read one: find your current on the bottom, go up until you hit the curve, go across to read the time. Real curves are drawn as bands — a minimum and a maximum clearing time — because no two devices are identical.

The sloped upper part is the long-time or inverse-time region: overloads, tripped by heat (a bimetal or fuse element) or an electronic model of it. The near-vertical drop is where the instantaneous element takes over — magnetic or electronic, no intentional delay. Adjustable breakers add a short-time region in between so an upstream device can deliberately wait a few cycles.

Two ratings, two questions

Every device has a trip rating (or setting): the current it is built to carry without tripping and above which it starts to act. It also has an interrupting rating: the most fault current it can safely break. They are unrelated — a small branch breaker can have a modest interrupting rating while a fuse of the same trip rating has a much higher one. If the fault current available where a device is installed is more than it can interrupt, a fault can destroy it rather than clear. The available fault current is covered in fourth year; the ratings required are in your code book.

Selective coordination

Coordination means only the device closest to the fault opens. On a curve sheet, the downstream band sits fully left of and below the upstream band at every current up to the fault current available there. Where bands overlap, both devices might open. Some systems — emergency and life-safety feeders, elevators, certain health-care circuits — are required to be coordinated; the code and the engineer decide which and to what level.

Worked: reading a practice curve (invented numbers)

A branch circuit protected by a device with a practice rating of 20 A is carrying 30 A. That is 30 ÷ 20 = 1.5 × its rating — an overload, the sloped part of the curve.
On the practice curve, 1.5 × sits where the band reads roughly one to several minutes. The device will not trip on a two-second motor start at that level; it will trip if the overload persists.
Now a bolted fault on the same circuit draws 2,000 A: 2,000 ÷ 20 = 100 × — far into the instantaneous region, so it clears in about a cycle.
The feeder upstream (practice curve) reads 0.3 s at 2,000 A. The branch clears in about 0.02 s: the branch opens first, so at this current the pair coordinates.
At 20,000 A both curves are in their instantaneous regions and the bands overlap — both could open. That is the region a coordination study pays attention to.

En la obra

Tips

Ask how it tripped: instantly on switch-on points to a fault; after running a while points to an overload.
Before resetting into a suspected fault, find it — insulation-test the circuit with the load disconnected.
A blown fuse on one phase of a motor circuit can leave it single-phasing; check all three and the motor.
Read the device’s nameplate for both numbers: its trip rating and its interrupting rating.
Replace like with like — a different breaker family or fuse class can have a very different curve.

Common mistakes

Treating a trip rating as the interrupting rating (or the reverse).
“Fixing” nuisance trips by putting in a bigger breaker — the conductor is now unprotected.
Resetting a breaker repeatedly into a dead short.
Assuming the main will not trip for a branch fault without anyone having checked coordination.
Swapping a fuse for a different class because it fits the clips.

Check yourself

0 right of 0 answered 7 questions · pick one answer each

1.What is the difference between an overload and a short circuit?
2.On a time-current curve, which way does "more current" run, and which way "more time"?
3.Why is the curve of an inverse-time breaker or fuse sloped rather than a straight vertical line?
4.What does "selective coordination" aim for?
5.You lay the branch breaker's curve and the feeder breaker's curve on one sheet. What indicates they are likely to coordinate at a given fault current?
6.A breaker trips instantly the moment a circuit is switched on, every time, with nothing plugged in. The most likely story is…
7.Why does a protective device also carry an interrupting rating?

Answers

B. An overload is normal-path current somewhat above rating for a while; a short circuit is current taking an unintended low-impedance path, many times the rating — An overload is too much load on a healthy circuit — the current is modestly high and the danger is heat over time. A short circuit (or ground fault) is current bypassing the load through a fault, often hundreds or thousands of amps, and the danger is immediate. A protective device is built to treat the two very differently.
B. Current across (left to right), time up (bottom to top) — both usually on logarithmic scales — Current is on the horizontal axis and time on the vertical one, both logarithmic so they can cover amps to thousands of amps and milliseconds to hours. Read across to your current, then up to the curve, then across to the time.
B. So it waits longer on small overloads (like a motor starting) and trips faster the bigger the current gets — Inverse time means the more current, the less time. A short surge — a motor or transformer inrush — rides through; a sustained overload trips after a while; a fault trips quickly. The instantaneous part at the bottom right handles faults with no intentional delay.
B. That only the device nearest the fault opens, so the rest of the system stays on — Coordination means the protective device closest to the fault clears it and the upstream devices do not. A short in one branch circuit should take out that branch breaker, not the feeder that also serves an elevator or an operating room.
B. At that current the branch device's curve (including its tolerance band) sits entirely below and left of the feeder device's band — the branch clears before the feeder starts to trip — Each device is drawn as a band (minimum to maximum clearing time). If the bands overlap at some current, both could open for a fault of that size. Clear space between them — branch fully faster than feeder — is what coordination looks like on paper. Whether a design must be coordinated, and to what level, is set by the code and the engineer.
B. A short circuit or ground fault on the circuit — An instant, repeatable trip with no load points to a fault — a nicked conductor, a screw through a cable, water in a box. An overload takes time to trip. Find the fault before resetting again: every reset into a dead short pushes fault energy through the circuit.
B. To say the most fault current it can safely break; a fault bigger than that can destroy the device instead of clearing — Tripping is not the same as interrupting. A breaker or fuse must be able to open the largest fault current available at its location. If the available fault current is higher than its interrupting rating it can fail violently. The available fault current and the ratings needed come from the fault study and the code book.

Preguntas frecuentes

Is a fuse better than a breaker?

They do the same job with different trade-offs. Fuses are simple, fast on big faults, and often have very high interrupting ratings, but they must be replaced and a single blown fuse can leave a three-phase motor single-phasing. Breakers reset, can carry adjustable trip settings and accessories, and open all poles together. Which is used where is a design decision within what the code allows.

Why does the motor overload relay not replace the breaker?

Because they protect different things. The overload relay protects the motor against running hot for too long and is deliberately slow. The breaker or fuses upstream protect the conductors and equipment against short circuits and ground faults. A motor circuit usually has both, sized by separate rules in your code book.

Where do I get time-current curves?

From the device maker — every breaker, fuse and relay family has published curves. Engineers overlay them in a coordination study, usually in software. On the job you mostly need to read one well enough to understand why a device did or did not trip.

Ver también

The overload relay (lab) — the slow half of motor protection
Ampacity — what the conductor protection is protecting
Available fault current
Corriente de falla de un transformador (barra infinita)
Protective relaying concepts
Leer el directorio de un tablero

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